Power Unit:

From the time Adam was created, man had been travelling across the different parts of the globe. Initally it was his own muscle that helped him. later, he trained animals to help him, afterwards the wind. As time went on, man, with his advancement technology was able to convert energy from one form to another. This machine which converts energy from one form to the other was named an engine.

By definition,An engine or motor is a machine designed to convert energy into useful mechanical motion.

Now coming to the case of an automobile,Generally the power unit of an automobile is an heat engine.Now-a-days electric motors are also used to provide driving force. Therefore the important types of power units are:

Heat engines:

Heat engines are those that convert the heat energy into mechanical work.it is also important for you to under stand the meaning of the process of combustion in heat engine context."Combustion" refers to burning fuel with an oxidizer, to supply the heat.Broader classification of engines:

Combustion heat engines: These type of engines use the heat produced during combustion processes to do work.

Non-Combustion heat engines: These type of engines convert heat from noncombustive processes into mechanical work(as in the case of a nuclear reactor)

External combustion engines:

Although they are not in use today, it still important for you to know at least their working.An external combustion engine (EC engine) is a heat engine where an internal working fluid is heated by combustion of an external source, through the engine wall or a heat exchanger. The fluid then, by expanding and acting on the mechanism of the engine produces motion and usable work. The fluid is then cooled, compressed and reused (closed cycle), or (less commonly) dumped, and cool fluid pulled in (open cycle air engine).

Internal combustion engines:

The internal combustion engine is an engine in which the combustion of a fuel (normally a fossil fuel) occurs with an oxidizer (usually air) in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine. This force is applied typically to pistons, turbine blades, or a nozzle. This force moves the component over a distance, transforming chemical energy into useful mechanical energy.

Before understanding how an IC engine works, it is important that you know its parts(components)..The important components of an IC engine are:

Cylinder block and the Cylinder,

Piston

Combustion chamber

inlet manifold

exhaust manifold

Inlet and exhaust valves

spark plugs or injectors

Connecting rods

Crank shaft

Piston rings

Gudgeon pin

Cam shaft and cams

Fly wheel

The image above shows the location of the various components in an IC engine.Cylinder block and the Cylinder: The cylinder block is the main supporting structure of an engine. Even in the cas eof a multicylinder engines, the cylindes are casted together as a single cylinder block. the cyliner head is mounted on the top of the cylinder block with the help of boltes. Both the cylinder and the cylinder block are provided with water jackets for cooling.The bottom part of this block is bolted with a cover called the crank case. This acts as the sump for lubricating oil and hence it is also called as sump.the center of the block is machined and bored accuretly to form the cylinder bore.

Piston: It is a cylindrical component fitted into the cylinder forming the moving boundary of the combustion system. It fits perfectly (snugly) into the cylinder providing a gas-tight space with the piston rings and the lubricant. It forms the first link in transmitting the gas forces to the output shaft.

Combustion Chamber:The space enclosed in the upper part of the cylinder, by the cylinder head and the piston top during the combustion process, is called the combustion chamber. The combustion of fuel and the consequent release of thermal energy results in the building up of pressure in this part of the cylinder.

Inlet Manifold:The pipe which connects the intake system to the inlet valve of the engine and through which air or air-fuel mixture is drawn into the cylinder is called the inlet manifold.

Exhaust Manifold:The pipe that connects the exhaust system to the exhaust valve of the engine and through which the products of combustion escape into the atmosphere is called the exhaust manifold.Inlet and Exhaust Valves:Valves are commonly mushroom shaped poppet type. They are provided either on the cylinder head or on the side of the cylinder for regulating the charge coming into the cylinder (inlet valve) and for discharging the products of combustion (exhaust valve) from the cylinder.

Connecting Rod:It interconnects the piston and the crankshaft and transmits the gas forces from the piston to the crankshaft. The two ends of the connecting rod are called as small end and the big end. Small end is connected to the piston by gudgeon pin and the big end is connected to the crankshaft by crankpin.

Crankshaft:It converts the reciprocating motion of the piston into useful rotary motion of the output shaft. In the crankshaft of a single cylinder engine there is pair of crank arms and balance weights. The balance weights are provided for static and dynamic balancing of the rotating system. The crankshaft is enclosed in a crankcase.

Piston Rings:Piston rings, fitted into the slots around the piston, provide a tight seal between the piston and the cylinder wall thus preventing leakage of combustion gases

Gudgeon Pin:It forms the link between the small end of the connecting rod and the piston.

Camshaft:The camshaft and its associated parts control the opening and closing of the two valves. The associated parts are push rods, rocker arms, valve springs and tappets. This shaft also provides the drive to the ignition system. The camshaft is driven by the crankshaft through timing gears.

Cams:These are made as integral parts of the camshaft and are designed in such a way to open the valves at the correct timing and to keep them open for the necessary duration.

Fly Wheel:The net torque imparted to the crankshaft during one complete cycle of operation of the engine fluctuates causing a change in the angular velocity of the shaft. In order to achieve a uniform torque an inertia mass in the form of a wheel is attached to the output shaft and this wheel is called the flywheel.

How engine works?

Working principle of a Four Stroke SI Engine

i. Suction or Intake Stroke: Suction stroke starts when the piston is at the top dead centre and about to move downwards. The inlet valve is open at this time and the exhaust valve is closed. Due to the suction created by the motion of the piston towards the bottom dead centre, the charge consisting of fuel-air mixture is drawn into the cylinder. When the piston reaches the bottom dead centre the suction stroke ends and the inlet valve closes.

ii. Compression Stroke: The charge taken into the cylinder during the suction stroke is compressed by the return stroke of the piston. During this stroke both inlet and exhaust valves are in closed position. The mixture that fills the entire cylinder volume is now compressed into the clearance volume. At the end of the compression stroke the mixture is ignited with the help of a spark plug located on the cylinder head. In ideal engines it is assumed that burning takes place instantaneously when the piston is at the top dead centre and hence the burning process can be approximated as heat addition at constant volume. During the burning process the chemical energy of the fuel is converted into heat energy producing a temperature rise of about 2000 °C. The pressure at the end of the combustion process is considerably increased due to the heat release from the fuel.

iii. Exhaust Stroke: At the end of the expansion stroke the exhaust valve opens and the inlet valve remains closed. The pressure falls to atmospheric level a part of the burnt gases escape. The piston starts moving from the bottom dead centre to top dead centre and sweeps the burnt gases out from the cylinder almost at atmospheric pressure.

The exhaust valve closes when the piston reaches T.D.C. at the end of the exhaust stroke and some residual gases trapped in the clearance volume remain in the cylinder. Residual gases mix with the fresh charge coming in during the following cycle, forming its working fluid. Each cylinder of a four stroke engine completes the above four operations in two engine revolutions, one revolution of the crankshaft occurs during the suction and compression strokes and the second revolution during the power and exhaust strokes. Thus for one complete cycle there’s only one power stroke while the crankshaft turns by two revolutions.

The Constant Volume or Otto CycleThe three cycles of great practical importance in the analysis of piston engine performance are:

i. The Constant Volume or Otto cycle

ii. The Diesel cycle

iii. The dual combustion or limited pressure cycle.

In this article we will describe the constant volume or Otto cycle illustrated in Fig. 2. The Otto cycle is the theoretical cycle for the spark ignition engine.In the air cycle analysis the induction and exhaust processes, represented by line 0-1 and 1-0 respectively, are neglected. The work done during both the processes is equal and opposite and hence cancels each other. After suction stroke 0-1, the piston is at bottom of outer dead center. This is represented by point 1 on p-V and T-s diagrams. The air is now compressed by reversible adiabatic process during the inward motion of the piston until the piston reaches top or inner dead center position (process 1-2).In this process entropy remains constant. At this time heat is added at constant volume so that the state of air changes from point 2 to 3. In an actual engine it is equivalent to burning of fuel instantaneously (by an electric spark) so that heat liberation is at constant volume. The next process is reversible adiabatic expansion3-4, piston moving outwards from top dead center to bottom dead center. At the end of this expansion process the heat is rejected by gases at constant volume, process 4-1, and the cycle is completed. In actual engine it corresponds to instantaneous opening of exhaust valve at 4 bringing down the pressure to atmospheric pressure 1, followed by exhaust stroke 1-0.

Since the processes 1-2 and 3-4 are adiabatic, no heat transfer takes place during these processes. The heat transfers are limited to addition of heat during the constant volume process 2-3 and rejection of heat during constant volume process 4-1.

Two-stroke Engine:As already mentioned, if the two unproductive strokes, viz., the suction and exhaust could be served by an alternative arrangement, especially without the movement of the piston then there will be a power stroke for each revolution of the crankshaft. In such an arrangement, theoretically the power output of the engine can be doubled for the same speed compared to a four-stroke engine. Based on this concept, Dugald Clark (1878) invented the two-stroke engine.

In two-stroke engines the cycle is completed in one revolution of the crankshaft. The main difference between two-stroke and four stroke engines is in the method of filling the fresh charge and removing the burnt gases from the cylinder. In the four-stroke engine these operations are performed by the engine piston during the suction and exhaust” strokes respectively. In a two-stroke engine, the filling process is accomplished by the charge compressed in crankcase or by a blower. The induction of the compressed charge moves out the product of combustion through exhaust ports. Therefore, no piston strokes are required for these two operations. Two strokes are sufficient to complete the cycle, one for compressing the fresh charge and the other for expansion or power stroke.

Figure 1 shows one of the simplest two-stroke engines, viz., the crankcase scavenged engine. The air or charge is inducted into the crankcase through the spring loaded inlet valve when the pressure in the crankcase is reduced due to upward motion of the piston during compression stroke. After the compression and ignition, expansion takes place in the usual way.

During the expansion stroke the charge in the crankcase is compressed. Near the end of the expansion stroke, the piston uncovers the exhaust ports and the cylinder pressure drops to atmospheric pressure as the combustion products leave the cylinder. Further movement of the piston uncovers the transfer ports, permitting the slightly compressed charge in the crankcase to enter the engine cylinder.

The top of the piston has usually a projection to deflect the fresh charge towards the top of the cylinder before flowing to the exhaust ports. This serves the double purpose of scavenging the upper part of the cylinder of the combustion products and preventing the fresh charge from flowing directly to the exhaust ports. The same objective can be achieved without piston deflector by proper shaping of the transfer port. During the upward motion of the piston from B DC the transfer ports close first and then the exhaust ports close when compression of the charge begins and the cycle is repeated.

Comparison of Four-stroke and Two-stroke EnginesThe two-stroke engine was developed to obtain a greater output from the same size of the engine. The engine mechanism also eliminates the valve arrangement making it mechanically simpler. Almost all two-stroke engines have no conventional valves but only ports (some have an exhaust valve). This simplicity of the two-stroke engine makes it cheaper to produce and easy to maintain. Theoretically a two-stroke engine develops twice the power of a comparable four stroke engine because of one power stroke every revolution (compared to one power stroke every two revolutions of a four-stroke engine). This makes the two-stroke engine more compact than a comparable four-stroke engine. In actual practice power output is not exactly doubled but increased by only about 30% because of

i. Reduced effective expansion stroke and

ii. Increased heating caused by increased number of power strokes that limits the maximum speed.

The other advantages of the two-stroke engine are more uniform torque on crankshaft and comparatively less exhaust gas dilution. However, when applied to the spark-ignition engine the two stroke cycle has certain disadvantages which have restricted its application to only small engines suitable for motor cycles, scooters, lawn mowers, outboard engines etc. In the SI engine, the incoming charge consists of fuel and air. During scavenging, as both inlet and exhaust ports are open simultaneously for some time, there is a possibility that some of the fresh charge containing fuel escapes with the exhaust. This results in high fuel consumption and lower thermal efficiency. The other drawback of two-stroke engine is the lack of flexibility, viz., the capacity to operate with the same efficiency at all speeds. At part throttle operating condition, the amount of fresh mixture entering the cylinder is not enough to clear all the exhaust gases and a part of it remains in the cylinder to contaminate the charge. This results in irregular operation of the engine. The two-stroke diesel engine does not suffer from these defects. There is no loss of fuel with exhaust gases as the intake charge in diesel engine is only air. The two-stroke diesel engine is used quite widely. Many of the high output diesel engines work on this cycle. A disadvantage common to all two-stroke engines, gasoline as well as diesel, is the greater cooling and lubricating oil requirements due to one power stroke in each revolution of the crankshaft. Consumption of lubricating oil is high in two-stroke engines due to higher temperature.

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